1. Field of the Invention
The invention relates to a MOS transistor with a ramped gate oxide thickness, a semiconductor device comprising a MOS transistor and a method for making a MOS transistor.
2. Discussion of the Background
Field effect transistors based on a metal-oxide semiconductor structure have revolutionized integrated circuit technology. However, conventional MOS transistors having a uniform gate oxide thickness across the length of the channel may exhibit high electric fields at the drain edge. These high electric fields can damage the drain region, especially an n-doped drain region. One source of damage associated with high electric fields is high-energy electrons or holes (referred to as hot electrons or hot holes), which can enter the oxide where they can be trapped, resulting in xe2x80x9coxide chargingxe2x80x9d. Over time, oxide charges will tend to gradually degrade the device performance, most notably by increasing the threshold voltage and decreasing the control of the gate on the drain current. Such damage can be fatal to the operation of a MOS device, and accordingly, a more reliable MOS device is sought.
Accordingly, one object of the invention is to provide a novel MOS transistor comprising (a) a substrate with a source region, a channel and a drain region, and (b) a gate comprising a gate material and a gate oxide having a source region edge and a drain region edge, wherein said gate oxide has a thickness greater at said drain region edge than at said source region edge.
Another embodiment of the present invention is directed to a circuit comprising the present MOS transistor.
Another embodiment of the present invention is directed to a method of making a transistor, having a gate oxide layer thicker at said drain region edge than at said source region edge.